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Driving High vs. Driving Drunk: We Still Have a Lot to Learn

February 19, 2018

By Troy Walden, Ph.D.

At more than 10,000 in 2015, the number of alcohol-related traffic deaths in the U.S. has remained relatively constant in recent years, representing nearly a third of all highway fatalities. The total tends to change by only a few percentage points from year to year. At roughly the same time, though, fatal crashes involving marijuana use seem to be spiking.

I say “seem to” because the statistics we have on pot-smoking drivers are far sketchier that what we have on drinking drivers, so it’s difficult to accurately detect any trends. We’ve been collecting drunk-driving data for decades, but we’ve been gathering numbers on cannabis-related crashes for only a few years, and in only a few states. A handful of studies help to illustrate how we still have much to learn:

Research by the AAA Foundation for Traffic Safety in 2016 said that the share of drivers in fatal crashes who had used marijuana more than doubled from 8 percent in 2013 to 17 percent in 2014. That’s significant, but it’s important to note that the study was limited to Washington state, where recreational pot use was legalized several years ago. No other states (pot-legal or otherwise) were examined.

A 2017 study published in the American Journal of Public Health looked at fatal crash data from 2009 to 2015 in Washington and Colorado, another marijuana-legal state. That research found no significant link between marijuana use and fatal crashes.

An analysis of non-fatal crashes by the Insurance Institute for Highway Safety showed a 3 percent increase in insurance claims from 2012 to 2016 in states where marijuana was legalized. 3 percent over 4 years may be meaningful, but it’s not staggering. Also, the pot-legal states were compared with neighboring states only; comparing them to states with similar populations or other characteristics may have yielded different results.

From 2013 to 2016, the share of drivers in Colorado from who tested positive for THC (the active ingredient in marijuana that causes impairment) grew by 145 percent. That’s an alarming percentage, but less so when you consider that the actual number of drivers grew from only 47 to 115.

I don’t in any way mean to suggest that impaired driving in any form should be dismissed as unimportant. Quite the contrary, in fact, and I do suggest that we should give the marijuana-impaired driving situation the attention it deserves. That begins with gaining a better understanding of the problem, and a better understanding begins with getting better information.

Let’s start by recognizing that in the world of crash statistics, marijuana use and marijuana impairment are different things. That’s because Tetrahydrocannabinol (THC), the active ingredient, doesn’t stay present in the body beyond a few hours. Conversely, Carboxy-THC, the inactive ingredient, can remain present – and detectable – in a person’s body for days after any use, long after the impairment effects are gone. Distinguishing between those two conditions isn’t possible with a routine urine test; a more involved (and costly) confirmatory blood test is necessary to determine whether active THC was present in the drivers blood at the time the specimen was drawn. The presence of active THC and performance on mental and physical tasks/testing (psycho-physical) together support a position of driver impairment and its contribution to crash causality. Unfortunately, the more expensive confirmatory test isn’t routinely performed unless officials find it necessary.

For instance, if the fatal crash involved only one car and marijuana use is suspected, officials may decide the test isn’t needed or doesn’t justify the additional cost. But if the crash involves multiple cars and victims (and associated criminal charges), the confirmatory test may be warranted to determine use by the driver and whether it is more likely than not, active THC in his/her system. In such cases, the confirmatory test is important not only for its data value, but also for its contribution to ensuring the result findings are robust so that justice may be served.

How many of these confirmatory tests are done across the United States? We don’t know, because such decisions are driven largely by individual enforcement and prosecutorial agencies, whose policies vary drastically depending on their geographic location, size, and funding constraints.

Our current source of reliable fatal crash numbers comes from the National Highway Traffic Safety Administration (NHTSA). The standard that NHTSA uses for classifying marijuana-related crash fatalities is “any detectable amount” – a vague, statistically low bar that contrasts sharply with the common and specific .08 blood alcohol content measurement for drunk driving. That leaves wide open the possibility that these crash deaths may be overrepresented in the data now available, and compromises our ability to fully grasp the extent of this problem.

Individual states where marijuana use is legal, on the other hand, present a mix of standards. Colorado and Washington have legal limits for TCH blood levels, but Oregon and Alaska don’t, instead relying on subjective interpretations by law enforcement officers.

Even with a consistent standard for THC intoxication, however, it’s difficult to ensure that such a standard would be universally fair. That’s partly because the human body doesn’t metabolize marijuana in the same way as alcohol, which helps to explain why police have a reliable tool – the breathalyzer – to help determine whether a driver is alcohol-impaired, but there’s no such tool to test for marijuana intoxication, at least not yet. Alcohol and cannabis are both drugs, but they’re very different drugs and they behave in different ways.

So, given all the data limitations and procedural challenges we face, what can we do? In addition to raising public awareness and maintaining high-visibility enforcement, the Substance Abuse and Mental Health Services Administration offers these recommendations:

Require the collection, testing and reporting of confirmatory/specificity blood specimens by toxicology laboratories in all drug-impaired driving cases.

Modify state crash records to require the capture and submission of confirmatory/specificity blood test drug results.

Obtain funding from state sources to support additional confirmatory/specificity blood testing in all drug related impaired driving cases, to support additional data warehousing of those test results in the state crash records systems, and support intensive supervision drug courts.

No one who lives in a motorized society can ignore the enormous human cost of traffic crashes. To make real progress, traffic safety professionals, especially those involved with impaired driving, will need to focus their efforts on developing safety treatments that not only are evidence-driven but also consider the health and safety benefits that each treatment has on affecting positive and progressive behavioral change.

But we can’t just continue to say that we need to do something. It’s equally important that we know the right thing to do. In the case of marijuana-impaired driving, it is essential that we raise questions about the nature, source, and quality of information that influences the decision making process for safety treatment development.

Troy Walden, Ph.D. is the Director of the Center for Alcohol and Drug Education Studies at the Texas A&M Transportation Institute; he is also a former law enforcement officer.